A rare-earth alloy sintered magnet (permanent magnet) is normally produced by compacting a powder of a rare-earth alloy, sintering the resultant powder compact and then subjecting the sintered body to an aging treatment. Permanent magnets currently used extensively in various applications include rare-earth-cobalt based magnets and rare-earth-iron-boron based magnets. Among other things, the rare-earth-iron-boron based magnets (which will be referred to herein as “R—Fe—B based magnets”, where R is one of the rare-earth elements including Y, Fe is iron and B is boron) are used more and more often in various electronic appliances. This is because an R—Fe—B based magnet exhibits a maximum energy product, which is higher than any of various other types of magnets, and yet is relatively inexpensive.
An R—Fe—B based sintered magnet includes a main phase consisting essentially of a tetragonal R2Fe14B compound, an R-rich phase including Nd, for example, and a B-rich phase. In the R—Fe—B based sintered magnet, a portion of Fe may be replaced with a transition metal such as Co or Ni and a portion of boron (B) may be replaced with carbon (C). An R—Fe—B based sintered magnet, to which the present invention is applicable effectively, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example.
In the prior art, an R—Fe—B based alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare-earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
Recently, a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, a molten alloy is brought into contact with, and relatively rapidly cooled by, a single chill roller, a twin chill roller, a rotating disk or the inner surface of a rotating cylindrical casting mold, thereby making a solidified alloy, which is thinner than an alloy ingot, from the molten alloy. The solidified alloy prepared in this manner will be referred to herein as an “alloy flake”. The alloy flake produced by such a rapid cooling process normally has a thickness of about 0.03 mm to about 10 mm. According to the rapid cooling process, the molten alloy starts to be solidified from its surface that has been in contact with the surface of the chill roller. That surface of the molten alloy will be referred to herein as a “roller contact surface”. Thus, in the rapid cooling process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid cooling process, has a structure including an R2Fe14B crystalline phase and an R-rich phase. The R2Fe14B crystalline phase usually has a minor-axis size of about 0.1 μm to about 100 μm and a major-axis size of about 5 μm to about 500 μm. On the other hand, the R-rich phase, which is a non-magnetic phase including a rare-earth element R at a relatively high concentration and having a thickness (corresponding to the width of the grain boundary) of about 10 μm or less, is dispersed on the grain boundary between the R2Fe14B crystalline phases.
Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be referred to herein as an “ingot alloy”), the rapidly solidified alloy has been quenched in a shorter time (i.e., at a cooling rate of 102° C./sec to 104° C./sec). Accordingly, the rapidly solidified alloy has a finer structure and a smaller crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly over the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has the above-described advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alternative alloy preparation method called “Ca reduction process (or reduction/diffusion process)” is also known in the art. This process includes the processing and manufacturing steps of: adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare-earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio; subjecting the resultant mixture to a reduction/diffusion treatment within an inert atmosphere; diluting the reactant obtained to make a slurry; and then treating the slurry with water. In this manner, a solid of an R—Fe—B based alloy can be obtained.
It should be noted that any small block of a solid alloy will be referred to herein as an “alloy block”. The “alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a rapid cooling process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
An alloy powder to be compacted is obtained by performing the processing steps of: coarsely pulverizing an alloy block in any of these forms by a hydrogen occlusion process, for example, and/or any of various mechanical milling processes (e.g., using a disk mill); and finely pulverizing the resultant coarse powder (with a mean particle size of 10 μm to 500 μm) by a dry milling process using a jet mill, for example.
The R—Fe—B based alloy powder to be compacted preferably has a mean particle size of 1.5 μm to about 6 μm to achieve sufficient magnetic properties. It should be noted that the “mean particle size” of a powder refers to herein a mass median diameter (MMD) unless stated otherwise. However, when a powder with such a small mean particle size is used, the resultant flowability, compactibility (including cavity fill density and compressibility) and productivity will be bad.
A powder made by a rapid cooling process such as a strip casting process (at a cooling rate of 102° C./s to 104° C./S), in particular, has a smaller mean particle size and a sharper particle size distribution that a powder made by an ingot casting process. Thus, the former powder is significantly inferior in flowability to the latter powder. Accordingly, the variation in the amount of the powder to be loaded into a cavity may exceed its allowable range or the fill density thereof within the cavity may become non-uniform. As a result, the variation in the mass or size of the resultant compact may exceed its allowable range or the compact may crack or chip. Furthermore, in that case, the magnetization directions of the compact cannot be sufficiently aligned by an aligning magnetic field, and the resultant sintered magnet exhibits low magnetic properties (such as its remanence).
According to the direction in which the aligning magnetic field is applied, the pressing and compacting methods to obtain compacts for magnets are roughly classifiable into the two types of: a parallel pressing method in which the aligning magnetic field is applied parallel to the pressing (or compressing) direction; and a perpendicular pressing method in which the aligning magnetic field is applied perpendicularly to the pressing direction.
Hereinafter, a pressing and compacting method for making a compact for an arched magnet will be described with reference to FIGS. 1(a) and 1(b). In FIGS. 1(a) and 1(b), the arrow B indicates the direction in which the aligning magnetic field is applied during the compaction process.
To improve the productivity and magnetic properties, the arched magnet 1a shown in FIG. 1(a) is obtained by once making and then cutting the sintered block 1b shown in FIG. 1(b). In the prior art, a compact to be processed into the sintered block 1b is obtained by the perpendicular pressing method. This is because the perpendicular pressing method makes it possible to press and compact the given powder without disturbing its magnetic field orientations. Thus, a magnet obtained by the perpendicular pressing method normally exhibits better magnetic properties than a magnet obtained by the parallel pressing method.
Meanwhile, yoke members are often provided in the vicinity of a die hole, which will define a cavity in a die made of a non-magnetic material, thereby concentrating the magnetic flux toward the inside of the cavity and increasing the strength of the aligning magnetic field. The yoke members are normally provided within 15 cm from the inner wall of the die hole as measured in the alignment direction. This arrangement is adopted because the higher the strength of the aligning magnetic fields within the cavity, the higher the remanence Br of the resultant magnet will be. If such a technique of increasing the in-cavity strength of the aligning magnetic field by using the yoke members is combined with the perpendicular pressing method described above, then a permanent magnet with even better properties can be produced.
In recent years, a fine powder with particle sizes (FSSS particle sizes) of 6 μm or less is often used to reduce the grain sizes of a sintered magnet. To align such fine powder particles, a stronger magnetic field than the conventional ones needs to be applied. However, if the in-cavity magnetic field strength is increased with the yoke members, the in-cavity magnetic field strength will have a non-uniform distribution, in which the magnetic field strength increases toward the end of the cavity in the alignment direction. Such a magnetic field strongly attracts the magnet powder in the cavity toward the yoke members. As a result, the apparent density of the magnet powder will be lower at the center of the cavity than at the end of the cavity. Particularly in the conventional static magnetic field pressing process, the aligning magnetic field starts to be applied at an early stage of the compacting and compressing process step (at which the powder still has so low a density as to move freely within the cavity), and therefore, the powder is easily distributed non-uniformly within the cavity. In that case, the powder that has been gathered toward the end of the cavity is pressed and shifted toward the center of the cavity as the upper punch is lowered to press the powder. In the meantime, the orientation directions are disturbed at both ends of the cavity. For these reasons, in the perpendicular pressing process to be carried out with the yoke members, the degree of alignment and density of the resultant powder compact easily become non-uniform, and the uniformity of the magnet performance tends to deteriorate excessively. Also, if the yoke members are provided in the vicinity of the cavity, the magnetic flux is concentrated but tends to be curved easily.
In order to overcome the problems described above, a primary object of the present invention is to provide a method for compacting a rare-earth alloy powder so as to produce a sintered magnet with uniform magnetic properties.